I believe self-reporting of accidents was a regulation that Google had to operate under. Funny enough, I just checked where they used to have their monthly accident reports but now it's just a Waymo landing page:
https://www.google.com/selfdrivingcar/reports/
Internet archive snapshot:
https://web-beta.archive.org/web/20161011014050/https://www....
Sample report (PDF) https://web-beta.archive.org/web/20161213211933/http://stati...